Our sages tell a story about a family who was accustomed to finding faults in other people's lineage, saying that so-and-so was an illegitimate child, etc. When the local Dayanim (rabbinical judges) heard the slander, they investigated the gossiping family's background, and discovered that they were not only illegitimate themselves, but the descendants of slaves.
Jewish law states (Shulchan Oruch, Even HaEzer 2:2) that a rabbinical court must be suspicious of one who finds a blemish in another person, namely, that the one who finds blemishes is indeed blemished in the same way. An old Hebrew expression says, Berosh haganav boer hakova, or the hat burns on the thief's head. In other words, dishonest people think that everyone is a crook.
In practical terms, those who see faults in others are actually looking at themselves. So, if the other guy appears to be a chimpanzee, then we ought to buy ourselves a bunch of bananas and a bag of peanuts. The faults we see in others are those we need to correct in ourselves.
courtesy of Lazerbrody.com